21Studi Trent. Sci. Nat., 94 (2014): 21-28
It is astonishing to note that, after over 200 years of
serious scientic studies on cheilostome bryozoans, there
are still prominent structures in the skeleton and polypi-
de whose function(s) remain poorly understood or even
unknown. This applies to, for instance, the papillae in
some Cribrilinidae, spicules in Thalamoporellidae, opercu-
lar glands, at least some types of avicularia (e.g. the small
suboral or peristomial ones), or, as a prominent structure
in the orice, the lyrula, which is the focus of the present
In addition to its use in species-level distinctions and
descriptions of the orice in certain ascophoran bryozoans
the lyrula is also a trait employed in higher-level systema-
tics. The typical anvil-shaped lyrula (Fig. 1) characterises
the umbonulomorph family Escharellidae and the lepralio-
morph superfamily Smittinoidea (cf. Hayward & Ryland
1999), the latter also including taxa with umbonuloid fron-
tal shields. Its presence in these and other remotely related
taxa suggests that the lyrula has evolved independently se-
veral times. Besides the typical anvil-shaped lyrula, other
differently shaped structures at the proximal orice and
peristomial margin that are usually termed denticle, mucro
or pseudolyrula, are present in an even wider range of taxa.
This raises a number of questions: how the term and cha-
racter trait of the lyrula may best be dened; how the lyrula
is formed during zooid ontogeny and where it is positioned;
and, above all, what function(s) the lyrula and other apertu-
ral structures may have.
The idea for this paper developed from the inability
of the present authors to answer any of these questions im-
mediately, partly because there is contradictory information
on certain aspects of the lyrula, such as its position relative
to the operculum. Confronting a number of younger and
even several eminent bryozoologists with the question on
what its function may be revealed that we were not the only
ones lacking this knowledge. Having developed a working
© Museo delle Scienze, Trento 2014
What, if anything, is a lyrula?
Björn BERNING1,*, Kevin J. TILBROOK2 & Andrew N. OSTROVSKY3,4
1 Upper Austrian State Museum, Geoscience Collections, Welser Str. 20, 4060 Leonding, Austria
2 Museum of Tropical Queensland, 70-102 Flinders Street, Townsville, QLD 4810, Australia
3 Department of Palaeontology, Faculty of Earth Sciences, Geography and Astronomy, Geozentrum, University of Vienna,
1090 Vienna, Austria
4 Department of Invertebrate Zoology, Faculty of Biology, St. Petersburg State University, Universitetskaja nab. 7/9, 199034
St. Petersburg, Russia
* Corresponding author e-mail: firstname.lastname@example.org
SUMMARY - What, if anything, is a lyrula? - Although the lyrula is a character trait that is often used in taxonomic descriptions of cer-
tain ascophoran bryozoan taxa, its function as well as its position relative to the operculum remained ill-dened ever since the term was
introduced in the late 19th century. The presence of a variety of other oricial and peristomial structures of different shape and position
has added to the terminological confusion. Here we show that, in contrast to the condyles that are positioned at the (disto)lateral orice
margins below the level of the operculum, all structures at the proximal margin are situated above the operculum, and that they all serve
the same ultimate function: to partition the orice and peristome into a distal part that is occupied by the tentacles, and a proximal part
through which water can freely ow into and out of the ascus during tentacle protrusion and retraction, respectively. Finally, we provide
amended denitions of the terms lyrula, central and lateral denticle, as well as peristomial groove and ridge.
RIASSUNTO - Cos’è una lirula, se è qualcosa? - Sebbene la lirula sia un carattere spesso usato nella descrizione tassonomica di certi
taxa di briozoi ascofori, la sua funzione così come la sua posizione rispetto all’opercolo, sono rimasti mal deniti n da quando il termine
fu introdotto nel tardo 19° secolo. La presenza di una varietà di altre strutture oriciali e peristomiali di differente forma e posizione si è
aggiunta alla confusione terminologica. Nel presente lavoro si mostra come, in contrasto ai condili che sono posizionati ai margini (disto)
laterali dell’oricio, sotto il livello dell’opercolo, tutte le strutture che si trovano lungo il margine prossimale siano situate sopra l’opercolo,
e come esse servano tutte a svolgere un’unica funzione: separare l’oricio e il peristoma in una parte distale, occupata dai tentacoli, e una
prossimale attraverso cui l’acqua può uire liberamente verso e dall’asco, rispettivamente durante la protrusione e la ritrazione dei tenta-
coli. Inne, vengono fornite denizioni emendate dei termini lirula, denticoli centrali e laterali così come di solco e cresta peristomiale.
Key words: Bryozoa, functional morphology, lyrula, orice
Parole chiave: Briozoi, morfologia funzionale, lyrula, oriciume
Bryozoan Studies 2013 - Antonietta Rosso, Patrick N. Wyse Jackson & Joanne Porter (eds)
22 Berning et al. What is a lyrula?
hypothesis of our own, we searched through the late 19th
and 20th century literature for further clues. It then became
clear that the answers to these questions were already out
there. Fortunately (for us at least), they are the same as tho-
se that we had come up with.
2. HISTORICAL BACKGROUND
Previously simply referred to as a tooth or denticle
(e.g. Busk 1854: 75), the term lyrula (diminutive of the
Greek lyra = lyre) was established by Jullien (1888: 52)
in the description of orice characters when introducing
the family Smittidae [sic]: “Orice suborbiculaire dont la
lèvre inférieure est entière et porte une dent médiane, lyrule
(lyrula), située au-dessus de l’opercule […]”. He was thus
very specic in stating that the median tooth, the lyrula, was
situated above the operculum. According to the glossary
provided by Bassler (1953: G12) in the Treatise on Inver-
tebrate Paleontology, the lyrula is a “Median, commonly
anvil-shaped tooth on proximal edge of orice, located be-
low the mucro […]”, whereas the mucro was considered to
be a “Rounded or spinelike projection at proximal edge of
peristome in some cheilostomes (Ascophora), located on
proximal side of lyrule […]”. It is a rather vague denition
and may imply that the lyrula is only present in species
with a peristome that bears a mucro. Arguably, in recent
times the denition given in Hayward & Ryland’s (1979:
300) Synopsis of the British Fauna had the greatest impact
owing to its wide use, and also because their denition for-
med the basis of the glossary of terms used on the Bryozoa
Homepage (Bock 2013): “Subopercular median tooth, of-
ten anvil-shaped, on the proximal side of the orice in some
Cheilostomata”. Thus, in contrast to Jullien’s original state-
ment, the lyrula was here supposed to be positioned below
the operculum. In Zabala & Maluquer’s (1988: 16) Illustra-
ted Keys for the Classication of Mediterranean Bryozoa,
as well as in the 2nd edition of the Synopsis (Hayward &
Ryland 1999: 391), the positional specier was deleted and
no indication as to the placement of the lyrula relative to
the operculum was given. Accordingly, in recent times it
has been unclear whether the lyrula is positioned above or
below the operculum, or whether its position is taxon-de-
pendent, although its exact position has occasionally been
mentioned (e.g. Soule & Soule 1973: 375).
Owing to its supposed position below the opercu-
lum, the lyrula was occasionally thought of as being so-
mehow involved as a hinge of the operculum. Yet even if
condyles are absent, the hypothetical presence of a subo-
percular lyrula would necessitate the existence of a cen-
tral notch in the operculum to allow inward tilting of the
proximolateral parts of the operculum during opening.
We are not aware that such a distinctly shaped operculum
exists. More importantly however, in ascophorans with um-
bonuloid and lepralioid frontal shields, the lyrula simply
cannot be positioned beneath the operculum for structural
reasons. The proximal opercular margin is continuous with
the membrane of the ascus, which continues underneath
the frontal shield that, in turn, forms the proximal apertural
margin, including the lyrula. All species with a lyrula in
the Escharellidae, Romancheinidae, Smittinidae and Petra-
liellidae that were sectioned by one of us (ANO) showed
this particular structure of the proximal orice margin and
frontal shield. Thus, as originally and correctly dened by
Jullien (1888), the lyrula is always positioned above the
operculum (Fig. 2; Soule & Soule 1972: g. 16).
What, then, is the function of the lyrula? In most
cases it was thought of as protecting the orice. Levinsen
(1909: 43) noted that “[…] its frequent presence in species
which have a membranous or slightly calcied operculum,
shows that it must be regarded as a protective formation”.
Bassler (1953: G217) also stated that the Phyllactellipori-
Fig. 1 - An archetypical lyrula in an early astogenetic zooid of Es-
charella cf. variolosa (Johnston, 1838) from the Mediterranean Sea.
Fig. 1 - Archetipo di lirula in uno zooide a stadio astogenetico
precoce di Escharella cf. variolosa (Johnston, 1838) del Mar Me-
Fig. 2 - Parasmittina sp. from the Red Sea with the operculum in
situ; the lyrula (lower centre) is produced by gymnocystal calci-
cation during formation of the orice margin and is positioned
directly above the proximomedian opercular edge.
Fig. 2 - Parasmittina sp. del Mar Rosso con l’opercolo in situ; la
lirula (centro, in basso) è prodotta da calcicazione della gimno-
cisti durante la formazione del margine dell’oricio ed è situata
immediatamente sopra il bordo opercolare prossimo-mediano.
23Studi Trent. Sci. Nat., 94 (2014): 21-28
dae have a “[…] well-developed peristomie bearing a pro-
tective organ (lyrula, mucron) for the compensatrix”.
However, if a predator can get past a calcied peri-
stome or long oral spines and, if present, a sclerotised oper-
culum, it is rather unlikely to be stopped by a comparatively
delicate structure such as the lyrula and/or any other apertu-
ral dentition or mucro. We do not want to entirely rule out
a protective function, particularly if the lyrula is large and
covering a large part of the orice (Fig. 3). In this case, the
constriction of the orice by the lyrula is very likely to im-
pede the penetration of the feeding organs of certain preda-
tors. Also, it has been pointed out (Waters 1889; Levinsen
1909; Harmer 1957) that many species with a lyrula in the
families Lepraliellidae, Smittinidae and Petraliellidae have
a weakly sclerotized, membranous operculum. Neverthe-
less, we believe the main purpose of the lyrula and similar
oricial and peristomial structures is a different one.
3. LYRULAE, PERISTOMIAL DENTICLES
AND SUBORAL MUCROS: DIFFERENT
STRUCTURES WITH A SIMILAR FUNCTION
Besides the typical anvil-shaped lyrula, a range of
other structures may occur at the proximal margin of, and
approximately level with, the primary orice in a great
number of unrelated umbonulomorph and lepraliomorph
taxa. Owing to their different shape and smaller size, the-
se have usually been termed teeth or denticles, such as in
the lepraliomorph genus Mucropetraliella (Fig. 4A) and in
the umbonulomorph Celleporaria (Fig. 4B). When revising
the Petraliellidae, Tilbrook & Cook (2005) distinguished
between supraopercular central and lateral denticles althou-
gh some of the central denticles imaged (e.g. their gures
9A, 11A) are, owing to their shape and position, lyrulae
by denition (i.e. anvil- or lyre-shaped). An unequivocal
attribution is less obvious in cases where there are up to
six small denticles of equal size along the orice margin as
in Petraliella dentilabris (Ortmann, 1892) (see Tilbrook &
Cook 2005: g. 5A). Moreover, there may be strong intra-
colonial variability in lyrula and denticle shape and num-
ber. For instance, in a single colony of Celleporaria aperta
(Hincks, 1882), a central lyrula is present if there are three
denticles whereas a pseudosinus is formed when there are
only two denticles (Fig. 4C).
Owing to the variability in number, position and
morphology, the lyrula and other peristomial structures
(and even the condyles previously) have thus occasionally
been treated as synonymous. In fact, even Jullien (1888:
55) confused matters by regarding the peristomial teeth
in the genus Exochella, which are situated well above the
primary orice and level with the median denticle, as iden-
tical with the cardelles (= condyles) in the Smittidae. In
bleached specimens it may indeed occasionally be difcult
to distinguish condyles from lateral denticles, particularly
if the condyles are indistinct and the lateral denticles are
situated slightly below the central one(s), as for example in
some species of the Petraliellidae (Fig. 4A; see also Stach
1936; Harmer 1957: 706-708). In situations such as these
an examination of the orice and frontal shield from below
may be necessary to shed light on their exact position (abo-
ve or below the operculum) and origin (distolateral margins
vs. proximal orice margin).
The function of the lyrula and supraopercular den-
ticles in the living zooid was rst proposed by Harmer
(1957: 693), although he did not explicitly mention the
lyrula in this regard (but see below). It is also apparent
from presumably the rst and only sketch of a lyrula in
conjunction with an open operculum drawn by Mary Ro-
gick, which was published in the same year (Rogick 1957:
pl. 1, g. 5). The image, which is reproduced here (Fig. 5),
shows that the operculum is stopped by the distal margin of
the lyrula in an approximately vertical position. Whereas
Rogick (1957: 1) cited Bassler’s notion that the lyrula is
a protective organ, Harmer (1957: 693) stated that the su-
praopercular lateral denticles “[…] restrict the movement
of the opercula, in order to provide for the free passage of
water through the lateral sinuses”. Harmer was well aware
that the lyrula was positioned above the operculum, as is
evident from the statement on p. 719 (“[…] the lyrula and
suboral avicularium are well raised above the depressed
frontal membrane, which is continuous with the base of the
operculum […]”), and referred to it as such or as a central
denticle. Unfortunately, Harmer (1957) did not provide a
glossary in which he could have claried his own views on
the true position and function of the lyrula.
Thus, on both sides of the lyrula (and in between
the lateral denticles, if these are present), space remains
free at the proximal margin of the primary orice through
which water can ow into the ascus while the tentacles are
protruded, and out of it when they are retracted. Structures
that are functionally analogous to the notches lateral to the
lyrula and between other apertural dentitions have evolved
in other cheilostome taxa in the form of an oricial sinus
and frontal shield ascopore.
Looked at from this perspective, all other structures
at the proximal orice margin described above (Fig. 4A-
C) also seem to enable free water ux as they prevent the
operculum from obstructing the proximal part of the ori-
ce, and, at the same time, provide pseudosinuses for wa-
ter compensation. There are other aberrant structures that
Fig. 3 - The extremely large lyrula in Parasmittina tropica (Waters,
1909) from the Red Sea probably also has a protective function.
Fig. 3 - La lirula estremamente grande di Parasmittina tropica (Wa-
ters, 1909) del Mar Rosso probabilmente ha anche una funzione
24 Berning et al. What is a lyrula?
may full the same ‘doorstop’-like function. For instance,
the suboral mucro in Hippomenella mucronelliformis (Wa -
ters, 1899) arches over the orice, and is positioned well
in reach of the operculum (Fig. 6A). The same may apply
to the single lateral denticle in Drepanophora species (Fig.
6B; see also Gordon 2000: g. 7), and to the mucro-like
prolongation towards the orice of the suboral avicularian
cystid, as for instance in certain species of Rhynchozoon
(Fig. 6C) or Smittoidea (Harmer 1902: 304).
In species with a relatively well-developed peristo-
me it can commonly be observed that additional denticles
are present along the proximal peristomial margin, even
if a lyrula is present just above the primary orice. These
structures, which are not situated directly at the level of
the orice but higher up in the peristome, i.e. at some ver-
tical distance from the operculum, are unable to arrest the
operculum but full the same general function of keeping
the proximal part of the aperture free of the tentacles (Che-
etham et al. 2007: 6). Examples are found in the genera
Metrarabdotos (Fig. 7A) and Parasmittina (Fig. 7B).
It should be stressed that in the absence of informa-
tion on the precise location and size of the operculum, i.e. in
bleached or fossil specimens, it may sometimes be difcult
to differentiate between a lyrula sensu strictu and a superpo-
sitioned peristomial denticle. Functionally identical structu-
res are indentations in the terminal peristomial aperture, as
in Saevitella peristomata (Waters, 1899) (see Berning 2012:
g. 17) or in the genus Escharoides (Fig. 7C).
Species with an extremely elongated peristome may
not only produce singular lyrulae or denticles but grooves
(Fig. 8A) or ridges (Fig. 8B), which sometimes extend
along the entire length of the peristome. Again, rather than
fullling defensive tasks, they also seem to serve the pur-
pose of keeping the tentacles free from the proximal mar-
gin. Particularly in species with a well-developed peristo-
me, the presence of such discrete passageways prevents
the water from interacting with the tentacles, and therefore
from creating turbulent ow and increased friction at the
polypide surface, which would be especially problematic
during rapid tentacle retraction and coeval escape of water.
Owing to the separation of the orice into a distal part that
is in its entirety occupied by the tentacles when protruding
and retracting, and a proximal area for waterow, the peri-
stome diameter can be kept at a minimum.
Indirect evidence supporting the tasks of the lyrula,
denticles and peristomial ridges/grooves is also offered by
species having long tubular peristomes that lack any inte-
rior structures, such as those in the genera Siphonicytara
or Margaretta (Fig. 9A). In these taxa, a partitioning of the
peristome is unnecessary because waterow takes place via
an ascopore. Similarly, lateral or proximal spiramina in the
peristome (which may occasionally terminate rather cen-
trally on the frontal shield, however) as in e.g. Tessarado-
ma (Fig. 9B; see also Gordon 2000: gs 17-20), Adeonella,
the Porinidae, and most taxa of the Gigantoporidae (Fig.
9C), full the same general function of water compensa-
tion, with the difference that the water is rst led into the
space above the orice at the base of the peristome, and
only then into the ascus. In exechonellid species with long
Fig. 4 - Oricial denticles. A. Central and lateral denticles in Mucropetraliella bennetti (Livingstone, 1926) from Heron Island (Great Bar-
rier Reef); note the indistinct condyles (arrows) at the distolateral orice margins. B. Two zooids of Celleporaria cf. aperta (Hincks, 1882)
from Oman with one central and two lateral denticles. C. Another zooid of C. cf. aperta in which the left of the lateral denticles have fused
with the central denticle to produce a single pseudosinus.
Fig. 4 - Denticoli oriciali. A. Denticoli centrali e laterali in Mucropetraliella bennetti (Livingstone, 1926) dell’Isola di Heron (Grande
Barriera); notare i condili indistinti (frecce) ai margini disto-laterali dell’oricio. B. Due zooidi di Celleporaria cf. aperta (Hincks, 1882)
dall’Oman con un denticolo centrale e due laterali. C. Un altro zooide di C. cf. aperta in cui il denticolo laterale sinistro è fuso con quello
centrale a formare un singolo pseudosinus.
Fig. 5 - View of an opened orice of Lageneschara lyrulata (Cal-
vet, 1909) from the inside, showing an operculum (O) that is
hinged on condyles (C) and stopped in a vertical position by the
lyrula (L) (modied after Rogick 1957).
Fig. 5 - Vista dall’interno di un oricio aperto di Lageneschara
lyrulata (Calvet, 1909), che mostra un opercolo (O) incernierato
sui condili (C) e bloccato in posizione verticale dalla lirula (L)
(modicato da Rogick 1957).
25Studi Trent. Sci. Nat., 94 (2014): 21-28
peristomes the water ows directly underneath the frontal
shield via foramina. Thus, in species that have other means
for water compensation, the formation of prominent struc-
tures in, and a partitioning of, the peristome is unnecessary.
To conclude, despite their wide range in size, posi-
tion and morphology, all supraopercular structures at the
proximal orice margin or in the peristome presumably
have the same overall function. However, a great inter-
specic, intraspecic and occasionally even intracolonial
variability in lyrula and denticle shape, size and number
suggests that their morphology is not of utmost importan-
ce. In fact, the lyrula and lateral or other denticles are not
indispensable, as is evident for example in the genus Cel-
leporaria in which species occur that lack any structures
at the proximal orice margin [on the Bryozoa Homepa-
ge, see e.g. Celleporaria agglutinans (Hutton, 1873) and
Celleporaria bispinata (Busk, 1854)]. Species in the genus
Pseudoustra (see Kuklinski et al. 2013) show an entire
Fig. 6 - Various structures at the proximal apertural margin that may allow for an unobstructed waterow during tentacle protrusion and
retraction, independent of the presence of a lyrula. A. A suboral umbo (arrow) overarching the orice that forms during ontogeny from the
distal frontal shield in Hippomenella mucronelliformis (Waters, 1899) from Madeira (photo: M.E. Spencer Jones). B. An asymmetrical
lateral denticle typical of Drepanophora spp. [the imaged specimen is Drepanophora tuberculata (Osburn, 1914) from Heron Island]. C.
The avicularian cystid of Rhynchozoon sp. from the Red Sea forming a mucro-like prolongation towards the orice.
Fig. 6 - Varie strutture del margine prossimale dell’apertura che consentono un usso d’acqua libero durante la protrusione e retrazione
dei tentacoli, indipendentemente dalla presenza di una lirula. A. Un umbone suborale (freccia) proiettato sull’oricio; si forma durante
l’ontogenesi dalla parte distale dello scudo frontale in Hippomenella mucronelliformis (Waters, 1899) da Madeira (foto: M.E. Spencer
Jones). B. Un denticolo laterale asimmetrico tipico di Drepanophora spp. [l’esemplare gurato è Drepanophora tuberculata (Osburn,
1914) dell’Isola di Heron]. C. Il cistide dell’aviculario di Rhynchozoon sp. dal Mar Rosso che forma una prominenza simile a un mucrone
Fig. 7 - Peristomial denticles. A. Interior view into the peristome of the fossil Metrarabdotos nehybai (Zágorsek et al., 2011); in addition
to the lower-level lateral and central denticles (which may have been in reach of the operculum), another pair of peristomial denticles is
present at a higher level, at which the two lower pseudosinuses merge into a single central one (photo: K. Zágorsek). B. Lateral peristomial
denticles in Parasmittina spondylicola Harmelin et al., 2009 from the Red Sea. C. Lateral and central peristomial denticles in Escharoides
longirostris Dumont, 1981 from the Red Sea.
Fig. 7 - Denticoli peristomiali. A. Vista interna del peristoma della specie fossile Metrarabdotos nehybai (Zágorsek et al., 2011); oltre ai
denticoli centrale e laterali situati a livello basso (che possono essere stati a contatto con l’opercolo), è presente un altro paio di denticoli
peristomiali situati a un livello più alto, in corrispondenza dei quali i due pseudoseni inferiori si fondono in un unico seno centrale (foto:
K. Zágorsek). B. Denticoli peristomiali laterali in Parasmittina spondylicola Harmelin et al., 2009 del Mar Rosso. C. Denticoli peristo-
miali laterali e centrali in Escharoides longirostris Dumont, 1981 del Mar Rosso.
26 Berning et al. What is a lyrula?
series of different orice morphologies: from a perfect
lyrula to a tiny hump and a straight proximal margin, to
the presence of a sinus (Fig. 9). Why some species have a
well-developed lyrula and lateral denticles whereas other
congeneric species exist without them remains yet to be in-
vestigated. Thus this notion, along with similar statements
made by many previous workers (e.g. Levinsen 1909; Har-
mer 1957), suggests that the character trait ‘lyrula’ should
be used with caution in interspecic and higher level sy-
4. TOWARDS A DEFINITION OF ‘LYRULA’
First and foremost, the lyrula and all other structures
at the proximal orice and peristome margin are, in contrast
to the condyles that originate from the distolateral orice
margins, positioned above the operculum. Thus, the distin-
ction between supraopercular denticles (sensu Tilbrook &
Cook 2005) on the one hand, and a subopercular lyrula (sensu
Hayward & Ryland 1979) on the other (see also the entries
Fig. 8 - Peristomial grooves and ridges. A. The proximal groove in the peristome of Parasmittina aff. spondylicola Harmelin et al., 2009
from the Red Sea is bordered by peristomial ridges that are formed by fusion of several superimposed lateral denticles. B. A central ridge
in the peristome of Teuchopora edwardsi (Jullien, 1883) from the north-east Atlantic.
Fig. 8 - Solchi e creste peristomiali. A. Il solco prossimale nel peristoma di Parasmittina aff. spondylicola Harmelin et al., 2009 del Mar
Rosso è delimitato da creste peristomiali formate dalla fusione di diversi denticoli laterali successivi. B. Una cresta centrale nel peristoma
di Teuchopora edwardsi (Jullien, 1883) dell’Atlantico nord-orientale.
Fig. 9 - Means of water compensation in species with peristomes that lack any prominent internal structures. A. Margaretta tenuis Harmer,
1957 from the Red Sea in which water compensation takes place via the ascopore. B. In Tessaradoma sp. from the Great Meteor Bank
the in- and outowing water is led into the space above the orice via the frontal spiramen (photo: B. Bader). C. In Gigantopora cf. pupa
(Jullien in Jullien & Calvet, 1903) from the Red Sea the water is led through a large window in the peristome.
Fig. 9 - Modalità per la compensazione dell’acqua in specie con peristoma senza strutture interne rilevate. A. Margaretta tenuis Harmer,
1957 del Mar Rosso in cui la compensazione dell’acqua avviene attraverso l’ascoporo. B. In Tessaradoma sp. del Banco Great Meteor
l’acqua uisce verso l’interno e verso l’esterno nello spazio sopra l’oricio attraverso lo spiramen frontale (foto: B. Bader). C. In Gigan-
topora cf. pupa (Jullien in Jullien & Calvet, 1903) del Mar Rosso l’acqua passa attraverso un’ampia nestra nel peristoma.
27Studi Trent. Sci. Nat., 94 (2014): 21-28
of ‘lyrula’ and ‘lateral denticle’ in the glossary on the Bryo-
zoa Homepage), is unjustied - both are above the level of the
operculum in the primary orice and are inferred to have the
functional roles outlined above. The vertical distance of denti-
cles from the orice, however, varies distinctly, from directly
above the operculum to the terminal peristomial aperture.
With regard to the origin of these structures, there also
seems to be no substantial difference between a central lyrula
or denticle and at least some of the lateral denticles (as for
example those seen in the Petraliellidae, cf. Tilbrook & Cook
2005). Moreover, even within a genus, the precise mode of
formation may differ (Soule & Soule 1972: 259). In both um-
bonulomorphs and lepraliomorphs they emerge from the gym-
nocystal calcication that forms either the proximal orice
margin (Soule & Soule 1972: g. 14), an outgrowth from the
vertical extension of the peristome, and/or, in some occasions,
the prolongation of a suboral avicularian cystid. As a structure
that may serve a similar function, suboral umbos may be pro-
duced by the distal part of the frontal shield during ontogeny
(e.g. Hippomenella), the surface of which may partly or enti-
rely be composed of skeleton with a cryptocystal fabric.
As to their function, some oricial structures may pro-
tect the bryozoan from predators that wish to gain access to
the polypide via the orice (cf. Berning 2008); in the case of
robust and heavily calcied structures (Fig. 3) this may often
be a major function. However, as most lyrulae and denticles
are relatively gracile, their function of partitioning the orice
for unobstructed waterow seems to be more important. This
hypothesis is corroborated by the presence of lyrulae even in
species with long, tubular and thickly calcied peristomes,
and by the absence of peristomial denticles from species that
have other means of water compensation, such as an ascopore
or peristomial/frontal spiramina and foramina.
Thus, a range of structures differing in shape and posi-
tion above the orice have the common overall task to facilita-
te unobstructed waterow. We therefore suggest not to restrict
the denition to a single term but to provide several context-
specic terms. Hence, all supraopercular oricial and peristo-
mial structures may be referred to the superordinate character
trait ‘denticle’, with speciers as to their position above the
orice if known. A lyre- or anvil-shaped central oricial denti-
cle may still be termed a ‘lyrula’, thereby implying its general
morphology. A lyrula may theoretically also occur higher up
in the peristome, in which case it would be termed a peristo-
mial lyrula. We suggest using the following terms:
denticle: any prominent structure in the orice or peristome
occurring above the operculum;
oricial denticle (lateral or central): structure within reach
of the operculum, with the ability to arrest it in a near-
peristomial denticle (lateral or central): structure in the pe-
ristome to prevent the tentacles from obstructing the
proximal peristomial margin;
lyrula: central denticle of lyre- or anvil-shape;
peristomial ridge/groove: longitudinal structures along the
proximal peristomial margin (occasionally a combina-
tion of both).
We would like to thank Beate Bader, Piotr Kuk-
linski, Mary Spencer Jones and Kamil Zágorsek for pro-
viding SEM images of several species, and a number of
bryozoologists for discussions on the lyrula. Joanne Porter
and two anonymous reviewers are thanked for comments
on the manuscript. ANO is grateful to the Russian Foun-
dation for Basic Research (RFFI) (grants 13-04-00758-a
and 13-04-10145-k) for funding. KJT acknowledges the
nancial support of the following organisations: Census of
Coral Reefs (CReefs), BHP Billiton, and the Queensland
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